The increasing life of telecommunications satellites and the changes in requirements associated with the various missions that can be entrusted upon them requires that payloads, in particular antennas, of future generations of satellites are flexible. This flexibility can be achieved at the geographic coverage area level of an antenna and/or at polarization level and/or at operating frequency band level. This flexibility provides the choice of several operating configurations of the antenna and the ability to modify, in orbit, the mission of the satellite.
Antennas placed on board satellites typically include geometrically shaped reflectors illuminated by a single source to cover extensive coverage areas pointed to on Earth. An antenna subsystem generally includes one transmission and reception antenna, or one transmission antenna and one reception antenna, for each coverage area. The geometric shape of the reflector can if necessary be defined so as to be optimized for several orbital positions of the satellite.
When the pointing directions aimed at are different, but the coverage shapes are similar, it is possible to place two sources side by side at the focal point of the reflector and to geometrically shape the reflector so as to obtain a compromise in performance between the two coverage areas. The spatial decoupling of the radiated beams between the two coverage areas is hence achieved by the angular distance separating the two spot beams illuminated by the two sources. Optimizing an antenna over several coverage areas degrades the directivity performance, this degradation able to exceed 1 dB when the sources are highly defocused, which, for a conventional architecture and one with given amplifiers, results in a reduction, by the same value, of the EIRP (Effective Isotropic Radiated Power).
Moreover, it is also possible to modify and orient the pointing of a spot beam on Earth by using small antennas with mechanical pointing. However, this requires all the elements of the antenna structure, notably the reflector and the sources, to be driven mechanically, which is complex to implement and requires the use of flexible waveguides.
A change in orientation of the linear polarization of the satellite antenna or a change from a linear polarization to a circular polarization can be achieved by using two sources, for example two horns, fed with linear and circular polarizations respectively and placed in front of an oversized reflector. The two sources are positioned as close as possible to the focal point of the reflector in order to reduce losses due to the defocusing of the sources and the consequential directivity losses of the antenna. Another possibility is the use of only one source connected to a complex electrical architecture combining two radiofrequency systems, the first operating in circular polarization and the second in linear polarization. This architecture leads to reliability problems, an increase in non-negligible ohmic losses related to the complexity of the RF system and a high cost of production.